All the cases that I know of involve smartphones. Do you have any links to stories involving feature phones?
BTW, some of the flip phones being sold today are Android Go (like the ZTE Cymbal T) or KaiOS (like Alcatel Go Flip), so they can have as much personal data collection as the standard smartphone. I assume that you want a feature phone (ie, “dumb phone”).
I’m kind of skeptical that the government is doing this with feature phones, because changing the ROM on a feature phone usually involves physically touching the phone, and it would be a lot of work to figure out how to add code to secretly turn on the microphone, because many of the feature phones had custom operating systems.
With a feature phone, you don’t have nearly as much data collection, since there is no GPS and often no camera, and you aren’t web browsing or doing other things that collect as much data. On the other hand, you are using a lot of insecure tech, like unencrypted SMS messaging. You want to use end-to-end encryption for your messaging and encrypted VOIP calls are more secure than normal cellular phone calls. If the government confiscates your feature phone, it wouldn’t be impossible to read the unencrypted data stored on its flash memory, whereas it would be much harder to get that data off an encrypted drive on the Librem 5.
I have to agree with kieran, the economics simply don’t work because people aren’t willing to pay that much for a feature phone. If you want a secure feature phone with hardware kill switches, disk encryption, end-to-end encryption in messaging, and encrypted VOIP calls, then you are going to end up paying close to the same amount as a smartphone.